1. Field of the Invention
The present invention relates to a variable resistor used in commercial equipment such as hearing aids, measuring instruments, communication devices, and sensors, and more particularly, to a small variable resistor.
2. Description of the Related Art
As a known example of such a variable resistor, Japanese Examined Patent Application Publication No. 5-59561 discloses a variable resistor wherein a resistor substrate on which terminals are mounted is insert-molded into a resin case, a rotor on which a slider is installed and an O-ring are accommodated in the case, the rotor is prevented from rising by placing a metallic cover on the case, and the cover is prevented from slipping off by engaging protrusions provided on the sides of the case with holes in leg portions provided on the cover.
As another known example of such a variable resistor, Japanese Unexamined Patent Application Publication No. 5-3108 discloses a variable resistor wherein a metallic cover is placed on a case from the upper portion thereof, and leg portions protruding from the cover are folded inwardly along the bottom surface of the case.
In each of these variable resistors, the resistor substrate is insert-molded into the case to provide heat resistance and superior sealing characteristics when the variable resistor is soldered to a printed circuit board. Hence, the resistor substrate (particularly, in the case of a ceramic substrate) is prone to cracking during molding, or molding resin may flow onto the surface of the substrate on which the resistor is provided, resulting in the formation of an insulating film thereon. Furthermore, in these variable resistors, it is necessary to take special steps to prevent the resin from intruding into the inside of the substrate, in order to prevent any insulating film from being located on the surface of the substrate. These problems result in reduced productivity and increased cost.
To overcome the above-described problems, preferred embodiments of the present invention provide a variable resistor which reliably seals the space between a rotor and a substrate without insert-molding the substrate into a case, and which resistor is produced at a greatly reduced cost.
Preferred embodiments of the present invention provide a variable resistor including a case which is open at the top and the bottom, a substrate which is fitted into the lower opening of the case, the substrate including a collector electrode on the top surface thereof and an arcuate resistor provided around said collector electrode, a rotor rotatably fitted into the upper opening of the case, a slider mounted on the bottom surface of the rotor and making sliding contact with the collector electrode and the resistor, an annular packing member disposed between the rotor and the substrate for sealing the space therebetween, and a metallic cover having a top plate portion for supporting the top surface of the rotor, a hole provided at the top plate portion such that a portion of the rotor is exposed, and a pair of leg portions extending downward along the sides of the case. The metallic cover supports the bottom surface of the substrate such that the metallic cover is disposed on the case from the upper portion of the case and the leg portions thereof are folded inwardly along the bottom surface of the case.
When assembling this variable resistor, first, the substrate is fitted into the lower opening of the case, and then the rotor is fitted into the upper opening of the case. It is preferable that the annular packing member be disposed on the top surface of the substrate and the slider be mounted on the bottom surface of the rotor in advance. Next, when putting the metallic cover on the case from the upper portion of the case, the pair of leg portions extend downwardly along the sides of the case. In this situation, a portion of the rotor is exposed from the window hole. The leg portions are folded inwardly along the bottom surface of the case and support the bottom surface of the substrate by the tip portions thereof. Thereby, the rotor is prevented from rising, the substrate is prevented from slipping off from the case, and the packing member and the slider are sandwiched between the rotor and the substrate. That is, a closed space is provided between the rotor and the substrate. By disposing the slider, the resistor, and the collector electrode, within this space, the intrusion of moisture and solder flux from the outside is prevented, which produces a variable resistor that achieves very stable performance.
Preferably, a spacer portion to maintain a desired spacing between the rotor and the substrate is provided on the inner surface of the case. Thereby, variations in assembly are avoided, and the compression allowance between the slider and the packing in the assembling process is uniform, and hence the electrical characteristics and the sealing characteristics of this variable resistor are very stable and uniform.
Furthermore, it is preferable that the packing member be directly applied on the top surface of the substrate, and on the outer peripheral side of the resistor. The packing member may instead be provided separately from the substrate and the rotor, and the packing may be disposed therebetween. However, in this case the packing member is prone to cause positional deviations, and it is difficult to maintain stable sealing characteristics. In contrast, the direct application of the packing onto the top surface of the substrate reliably prevents positional deviations thereof.
Moreover, guide grooves for guiding the leg portions of the metallic cover are preferably provided on the outer side-surfaces of the case. This stabilizes the positioning of the cover on the case, and facilitates the assembly thereof.
The features, characteristics, elements and advantages of the present invention will be clear from the following detailed description of preferred embodiments of the invention in conjunction with the accompanying drawings.